Rotary drill bits are commonly used for drilling bore holes or wells in earth formations. One type of rotary drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which typically includes a plurality of cutting elements secured to a face region of a bit body. Referring to FIG. 1, a conventional fixed-cutter earth-boring rotary drill bit 100 includes a bit body 102 that has generally radially projecting and longitudinally extending wings or blades 104, which are separated by junk slots 106.
A plurality of cutting elements 108 is positioned on each of the blades 104. Generally, the cutting elements 108 have either a disk shape or, in some instances, a more elongated, substantially cylindrical shape. The cutting elements 108 commonly comprise a “table” of super-abrasive material, such as mutually bound particles of polycrystalline diamond, formed on a supporting substrate of a hard material, conventionally cemented tungsten carbide. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutting elements or cutters. The plurality of PDC cutting elements 108 may be provided within cutting element pockets 110 formed in rotationally leading surfaces of each of the blades 104. The PDC cutting elements 108 may be supported from behind (taken in the direction of bit rotation) by buttresses 112, which may be integrally formed with the bit body 102. Conventionally, a bonding material such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements 108 to the bit body 102.
The bit body 102 of a rotary drill bit 100 typically is secured to a hardened steel shank having an American Petroleum Institute (API) thread connection 114 for attaching the drill bit 100 to a drill string (not shown). The drill string includes tubular pipe and component segments coupled end to end between the drill bit and other drilling equipment at the surface. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit within the bore hole. Alternatively, the shank of the drill bit may be coupled to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit, alone or in combination with rotation of the drill string from the surface.
During drilling operations, the drill bit 100 is positioned at the bottom of a well bore hole and rotated. Drilling fluid is pumped through the inside of the bit body 102, and out through the nozzles 116. As the drill bit 100 is rotated, the PDC cutting elements 108 scrape across and shear away the underlying earth formation material. The formation cuttings mix with the drilling fluid and pass through the junk slots 106, up through an annular space between the wall of the bore hole and the outer surface of the drill string to the surface of the earth formation.
The bit body 102 of a fixed-cutter rotary drill bit 100 may be formed from steel. Such steel bit bodies are typically fabricated by machining a steel blank (using conventional machining processes including, for example, turning, milling, and drilling) to form the blades 104, junk slots 106, pockets 110, buttresses 112, and other features of the drill bit 100.
As previously described, the cutting elements 108 of an earth-boring rotary drill bit often have a generally cylindrical shape. Therefore, to form a pocket 110 for receiving such a cutting element 108 therein, it may be necessary or desirable to form a recess into the body of a drill bit that has the shape of a flat-ended, right cylinder. Such a recess may be machined into the body of a drill bit by, for example, using a drilling or milling machine to plunge a rotating flat-bottomed end mill cutter into the body of a drill bit along the axis of rotation of the cutter. Such a machining operation may yield a cutting element pocket 110 having a substantially cylindrical surface and a substantially planar inner end surface for disposing and brazing a generally cylindrical cutting element 108 therein.
In some situations, however, difficulties may arise in machining such generally cylindrical cutting element pockets. For instance, there may be physical interference between the machining equipment used, such as a multiple-axis milling machine, and the blades of the drill bit adjacent to the blade on which it is desired to machine a cutting element pocket. This is particularly true when cutting element pockets are to be formed in the center, or “cone” region, of the bit face. As illustrated in FIG. 2, attempting to machine a cutting element pocket in blade 204 at a low angle and in the direction of the arrow may not be possible because of interference with blade 206. More specifically, the interference caused by blade 206 may inhibit the use of a desired machining path for a machining tool that is aligned generally along the axis of rotation thereof because at least one of the machining tool and the collet or chuck that retains the machining tool may contact adjacent blade 206. As a result, in order to form the desired cutting element pocket by way of a flat-bottomed machining tool, such as an end mill, the machining tool may be required to remove a portion of adjacent blade 206.
As a result of such tool path interference problems, it may be necessary to orient one or more cutting element pockets on the face of an earth-boring rotary drill bit at an angle that causes the cutting element secured therein to exhibit a back rake angle that is greater than a desired back rake angle. A lower, or more aggressive, back rake angle than that conventionally obtainable using the foregoing machining technique may be preferred to improve the rate of penetration while drilling.
Methods for overcoming such tool path interference problems have been presented in the art. For example, U.S. Pat. No. 7,070,011 to Sherwood, Jr., et al. discloses steel body rotary drill bits having primary cutting elements that are disposed in cutter pocket recesses that are partially defined by cutter support elements. The support elements are affixed to the steel body during fabrication of the drill bits. At least a portion of the body of each cutting element is secured to a surface of the steel bit body, and at least another portion of the body of each cutting element matingly engages a surface of one of the support elements.
However, there is a continuing need in the art for methods of forming cutting element pockets on earth-boring rotary drill bits that avoid the tool path interference problems discussed above and that do not require use of additional support elements.